1. Field of the Invention
The present invention relates to a polymer electrolyte membrane fuel cell (PEMFC) including a complex catalyst and a method for producing the complex catalyst. More specifically, the present invention relates to a polymer electrolyte membrane fuel cell including a complex catalyst whose catalytic activity is further enhanced even when the catalyst is poisoned by phosphoric acid, and a method for producing the complex catalyst.
2. Description of the Related Art
In recent years, new alternative energy sources to petroleum have attracted increasing attention as solutions to environmental pollution and energy depletion problems. Fuel cells using hydrogen or hydrocarbon fuels are considered the most promising means of alternative energy. Fuel cells are new power generation systems in which a fuel gas electrochemically reacts with an oxidant gas to create energy, which is directly converted into electrical energy. Considerable research efforts have concentrated on the development of fuel cells with higher energy efficiency and reliability.
Fuel cells are classified according to their structural features and operational environments. Representative examples of fuel cell types include molten carbonate type fuel cells (MCFCs), alkaline fuel cells (AFCs), solid oxide fuel cells (SOFCs), direct methanol fuel cells (DMFCs), and polymer electrolyte membrane fuel cells (PEMFCs).
Of these, polymer electrolyte membrane fuel cells (PEMFCs) are simpler in structure than other fuel cells and exhibit rapid start-up, fast response, and good durability. PEMFCs can use methanol or natural gas as a fuel other than hydrogen. Due to these advantages, the applicability of PEMFCs is extended to power sources of vehicles, distributed on-site power generation facilities, power sources for military use, power sources for spacecraft, and electric power for household use.
Such a PEMFC generates electricity based on the following mechanism. Hydrogen as a reaction gas is supplied from an anode. The hydrogen molecules are oxidized into hydrogen ions and electrons. The hydrogen ions are delivered to a cathode via a polymer electrolyte membrane. Oxygen molecules accept the electrons at the cathode. As a result of the reduction, the oxygen molecules are converted into oxygen ions. The oxygen ions react with the hydrogen ions moving from the anode and are converted into water molecules. Through such a series of reactions, a potential is created between the anode and the cathode to generate electricity. The series of reactions are schematically depicted below.
Anode: H2→2H++2e−
Cathode: 1/2O2+2H++2e−→H2O
Overall cell reaction: H2+1/2O2→H2O
The cathode includes a catalyst that catalyzes the reduction of oxygen. Since a high temperature PEMFC using a phosphoric acid-doped polyimidazole electrolyte membrane has a mechanism that is not dependent on water, an operating environment of 100° C. or higher can be set. Therefore, the high temperature operating environment can at least overcome the phenomenon of cathode catalyst poisoning by carbon monoxide, which is a serious problem inherent to perfluorosulfonic acid (PFSA) polymer electrolyte membranes and Nafion membranes (DuPont) as commercially available products that are widely in use. This enables the use of various fuels such as reformed gases in the high temperature PEMFC.
In addition, the high temperature PEMFC can contribute to a reduction in the size of a system because the high temperature PEMFC can use compact reformer due to tolerance to CO poisoning, requires no humidifier and the like and is advantageous in waste heat utilization. However, phosphoric acid from the phosphoric acid-doped polyimidazole electrolyte membrane poisons the cathode catalyst. This catalyst poisoning was found to cause an increase in the amount of a noble metal catalyst such as platinum despite high operating temperature. Catalyst poisoning by phosphoric acid in the high temperature PEMFC using the phosphoric acid-doped polyimidazole electrolyte membrane is considered as the most urgent task to solve.